line-range-format.txt: clarify -L:regex usage form

line-range-format.txt: clarify -L:regex usage form

blame/log documentation describes -L option as:

  -L<start>,<end>
  -L:<regex>

  <start> and <end> can take one of these forms:

    * number
    * /regex/
    * +offset or -offset
    * :regex

which is incorrect and confusing since :regex is not one of the valid
forms of <start> or <end>; in fact, it must be -L's lone argument.

Clarify by discussing :<regex> at the same indentation level as "<start>
and <end>...":

  -L<start>,<end>
  -L:<regex>

  <start> and <end> can take one of these forms:

    * number
    * /regex/
    * +offset or -offset

  If :<regex> is given in place of <start> and <end> ...

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>